Containment vessels are used for the evaporation of materials, for example, to deposit thin films on substrates. Typically, such containment vessels are crucibles capable of withstanding high temperatures for the vaporization of the contents of the crucible. In the photovoltaic industry, for example, materials such as copper, aluminum, indium, gallium or selenium are vaporized and deposited as a thin film coating on a substrate.
The crucibles are heated by conventional means such as electrical resistance heating coils, induction heating and the like to provide an evaporation system. The crucibles can be arranged in an array below the substrate, which is coated by the vaporized material us it is passed over the top of the evaporation system. The system is operated under a sufficiently low vacuum to enable evaporation of the metals. Many of the techniques are similar to those used for molecular beam epitaxi (“MBE”) which has a sufficiently low vacuum to provide a molecular beam flow from the crucible to the substrate.
Ideally, the crucibles will have several desirable features. They should be resistant to the corrosive action of the molten materials which they hold, and to the metal vapors. They should be stable up to about 1,800° C., and in vacuum. They should be easy to charge with a sufficient volume of film-forming materials, easy to heat, and they should have a top opening geometry engineered to control the pattern of vapor flux flowing from the crucible(s) to the substrate. This is true for MBE and is well documented in the literature, and, by logical extension, is needed in the photovoltaic industry as well.
Typically the crucibles are made from such materials as hot pressed boron nitride (hpBN), pyrolytic boron nitride (pBN), and graphite (particularly, graphite coated with pBN).
Crucibles can take various forms, but are generally cylindrical or conical, on the order of 10 mm in diameter and 20 mm in length up to about 100 mm in diameter and 400 mm in. length. Larger size crucibles and heaters are advantageous for photovoltaic needs, with standard size production panels typically at 1,200 mm×600 mm.
Historically, MBE crucibles have been open on the top, with substantially straight cylindrical sidewalls, or very open in a conical form. In part, this is to help control the “beam” flow. The shape of the top of the crucible impacts the deposition profile, and impacts the stability of the source material. A conical exit cone seems to be preferred in the MBE industry.
MBE cells are often used at an angle of about 45°, so an open, conical shape may not hold sufficient volume of material. When tipped at an angle, an open cone will pour out its contents. A one-piece, integral crucible, with a large body and a narrow orifice is known in the art. This design is available under the designation SUMO® from Veeco Instruments Inc.
For many metal sources, the material of choice for the crucible is a pyrolytic boron nitride (PBN). PBN is a material produced from chemical vapor deposition (CVD) on a graphite mandrel. To make a narrow orifice crucible part, the narrow orifice is machined into a graphite mandrel, the mandrel is then CVD coated with pBN, then the mandrel is oxidized out of the body of the crucible.
U.S. Pat. No. 4,812,326 discloses an evaporation source having a two-piece design. The evaporation material is vaporized and jetted through a nozzle having a flared opening to control the size of the atom clusters of the vapor jet.
WO96/35091 and WO98/08780 disclose unibody monolithic, one-piece negative draft crucibles for use in MBE effusion cells.
There yet remains need for improvements in crucible construction to accommodate the MBE process and other thermal evaporation processes as used in photovoltaic production.